Mechanocapillary Forming of Filamentary Materials.

Abstract

The hierarchical structure and organization of filaments within natural materials determine their collective chemical and physical functionalities. Synthetic nanoscale filaments such as carbon nanotubes (CNTs) are known for their outstanding properties including high stiffness and strength at low density, and high electrical conductivity and current carrying capacity. Ordered assemblies of densely packed CNTs are therefore expected to enable the synthesis of new materials having outstanding multifunctional performance. However, current methods of CNT synthesis have inadequate control of quality, density and order. In pursuit of these needs, a new technique called capillary forming is used to manipulate vertically aligned (VA-) CNTs, and to enable their integration in applications ranging from microsystems to macroscale functional films. Capillary forming relies on shape-directed capillary rise during solvent condensation; followed by evaporation-induced shrinkage. Three-dimensional geometric transformations result from the heterogeneous strain distribution within the microstructures during the vapor-liquid-solid interface shrinkage. A portfolio of microscale CNT assemblies with highly ordered internal structure and freeform geometries including straight, bent, folded and helical profiles, are fabricated using this technique. The mechanical stiffness and electrical conductivity of capillary formed CNT micropillars are 5 GPa and 104 S/m respectively. These values are at least hundred-fold higher than as-grown CNT properties, and exceed the properties of typical microfabrication polymers. Responsive CNT-hydrogel composites are prototyped by combining isotropic moisture-induced swelling of the hydrogel with the anisotropic stiffness of CNTs to induce reversible self-directed shape changes of up to 30% stroke. Centimeter scale sheets are fabricated by mechanical rolling and capillary assisted joining of CNTs. The mechanical stiffness, strength and electrical conductivity of CNT sheets are comparable to those of continuous CNT microstructures; and can be tuned by engineering the morphology of the CNT joints. Finally, the applicability of mechanocapillary forming to other nanoscale filaments is demonstrated using silicon nanowires synthesized by metal assisted chemical etching. Further work using the methods developed in this dissertation could enable applications such as directional liquid transport, adhesives, and biosensors; toward an end goal of creating multifunctional surfaces having arbitrary structural, interfacial, and optical responsiveness.